Specific regulation of immune response is our objective. To this end we have developed a rat renal allograft model. Two procedures, 1) intravenous injection of the recipient with donor antigen and 2) injection of the recipient with recipient type antibody against donor antigens, each produce specific suppression of graft rejection. When the two procedures are used together, grafts may survive and function as well as syngeneic grafts. Obviously an understanding of the mechanism of suppression might permit generalization of the procedures for application to clinical problems. Therefore, we have developed in vitro assays of cell-mediated immunity. We have determined that each procedure probably suppresses by a different mechanism and that rat antibody directed against recipient receptors for donor antigen block cell-mediated immunity in vitro. We are proceeding to determine whether such antibody promotes graft survival and whether such antibody plays an important role in the homeostasis of immune responses. We believe these findings have obvious significance for developing clinical procedures for regulating immune response: for transplants, for control of some autoimmune diseases, and possibly for control of growth and spread of some tumors.